1. Field of the Invention
The present invention pertains to fire detection systems and more particularly to fire detection systems that are capable of detecting conditions that are indicative of imminent combustion.
2. Related Prior Art
Fire protection regulations in the United States focus on two basic issues, protection of people and protection of buildings. The fire protection industry, who helps form the regulations, also focuses on these two issues with the primary focus on protection of people. In the secondary issue of protecting buildings, the protection of the contents of the building has been traditionally secondary and not as high a priority as the building. It is assumed that the contents of the building are protected by conventional fire systems. Since the value of the contents of a building is usually much less than that of the building, fire insurance coverage is relied upon as the most cost effective way to deal with this problem.
In most situations the focus on building safety is appropriate. However, in some instances, building protection is not where emphasis should be placed. Exceptions to the building first type of focus include high tech facilities where a high concentration of extremely expensive equipment is located or where service interruption is highly undesirable, such as a key defense facility. In general, these facilities are unmanned and personnel protection is not a consideration. Although personnel protection is always the highest priority, in the listed exceptional situations the integrity and operation of the building contents become more important than the building. In addition, this type of equipment is often significantly more vulnerable to fire damage from even small sized fires.
A variety of approaches have been used to solve this problem, Incipient fire detection systems of varying levels of sophistication have been built by a number of facilities. The commercial VESDA (Very Early Smoke Detection Apparatus) system is an example of a smoke/aerosol based early warning system which is widely used. In a different approach, Fermi National Laboratory has designed and currently operates an early warning system based on an evolved gas signature in order to protect the CDF detector.
Incipient detection systems of this type are completely site specific. An analysis of the likely ignition scenarios and of the combustible fuels in a given location is performed first. When the most likely fuel is identified, a literature search is performed to try to determine what gasses are given off by this material when it is heated. Typically there are several gasses that are given off and detectors for these specific gases are obtained. The detectors are then either mounted in the area of interest or mounted at the output end of a gas sampling system which samples the areas of interest. Detection of these gases in coincidence triggers a fire alarm.
Prior art systems have several weaknesses, some of which are listed as follows.
Initially, the prior art systems are completely specific to one fuel. If a second source of fuel is present and heated or ignited, the system could be potentially blind to this fuel if the gases given off are different than the ones for which the system is designed. Upgrading the system to detect a new set of gases will increase the number of gas detectors required. In a sequential gas sampling system, this may affect the sampling time at each point depending on the time response of the new detectors. The overall cost of the revised system increases a new gas detectors are added and maintained.
Another weakness is that the transport efficiency of some gases through sampling systems, for example, HCl gas, is known to depend strongly on the sampling tube type and length. For a system with two different tube lengths the gas mixture from a given combustible may be quite different coming from one monitored area than from another.
The precise species and relative concentrations of gases given off and transported through the sampling system depends on the rate of heating and other environmental effects such as humidity.
An additional weakness is that for a given combustible, the available literature may not include all the gases evolved when the material is heated, particularly when it is heated at different rates. Little or no information on unusual combustibles may be available. Therefore, a crucial signature of a fire precursor may be designed out of a system from the start.
Another weakness exists in the entire method. The entire method is predicated on the early apprehension of gases above their ambient concentrations. The time constants for existing electrochemical gas detectors and detectors employing optical filters are typically greater than the gas transit time through long sampling tubes in practical systems. The detector response time is therefore the main delay in any system employing sequential sampling of mote than a few locations, and the overall system delay grows with the number of sampling points.
In prior art systems the overall fire protection system cannot easily be packaged into a commercial product. The system built at one site is not likely to be optimal or adaptable to a different site, unless the combustible inventory and the spatial layout of the two systems are nearly identical. Each individual system is basically a large scale research project.
Examples of prior art systems are illustrated in the following U.S. Patents, all of which have one or more the foregoing deficiencies.
U.S. Pat. No. 5,519,382, titled "Mobile Fire Detector System", issued to Tim E. Pope et al., relates to a mobile fire detecting system that detects fire conditions st their incipient stage prior to the presence of visible smoke. This system contains an air sampling fire detector mounted to a cart supported by wheels. A tubular network in communication with the air sampling detector has a manifold enclosed by a box with four sensing hoses releaseably connected to the lateral outlets of the manifold and a telescoping mast supporting the manifold to position it at selectable heights. Four sampling heads are connected to the other ends of the sensing hoses and are peripherally spaced equally so that the maximum area is sampled for air to detect fire conditions.
U.S. Pat. No. 5,475,222, titled "Ruggedized Gas Detector", issued to John D. King, relates to a gas detector having two or more perforated concentric cylinders and having a concentric screen inside the innermost cylinder. A pair of perforated tubes are positioned inside the perforated screen, and an infrared light source is placed adjacent one end of one of the tubes and an infrared sensor is placed adjacent the other tube. A pair of inclined mirrors are positioned adjacent the respective other ends of the two tubes, and an optical light path is created from the IR source through a first tube, reflected by the mirrors to a return optical path through the second tube and ultimately to the IR sensor.
U.S. Pat. No. 4,264,209, titled "Gas Detector", issued to Arthur E. Brewster, relates to an apparatus for illuminating a gas or gas mixture and filtering the output thereof alternately with two filters. One filter has a passband at an absorption band of a gas to be detected. The other filter has a passband outside the absorption band.
U.S. Pat. No. 5,341,214, titled "NDIR Gas Analysis Using Spectral Ratioing Technique", issued to Jacob Y. Wong, relates to an instrument for determining the concentration of a particular gas that might be present in a sample using a waveguiding structure to serve both as an optical element and as a sample chamber. As an optical element, the waveguiding structure collects radiation from a black body source located at the entrance end of the waveguiding structure and conducts the radiation through the waveguiding structure, concentrating it on two infrared detectors mounted at the opposite end of the waveguiding structure. As a sample chamber, the waveguiding structure causes the radiation to undergo multiple reflections that result in the average path length being substantially greater than the physical length of the waveguiding structure. Each of the detectors has its own optical filter, and baffling assures that each detector responds only to radiation which has passed through its filter. One filter defines a spectral passband that coincides with the infrared absorption band of the gas to be measured. The other filter defines a non-absorbing or neutral passband. The electrical signals produced by the detectors are processed to provide a ratio, the value of which is related to the concentration of the particular gas to be detected.